Polymers have been widely investigated in the coatings industry for a number of applications, such as adhesives, sealants, clear coats, powder coats, and low adhesion backings. With appropriate choice of polymer backbone, they can be applied to a wide variety of surfaces. For example, acrylates typically have good adhesion to polar surfaces, and exhibit useful mechanical properties and good weatherability over a wide range of temperatures. Processability and final properties of polymers can often simultaneously be enhanced by a post-processing reaction, such as crosslinking. As a result, this has been an area of much interest and research.
High molecular weight polymers (Mn>100 kg/mol) can be coated through the use of solvent. These materials can subsequently be crosslinked through the addition of polyfunctional crosslinking agents that react with functionalities in the polymer, as described in Japanese Kokoku No. 58[1983]-046236. Alternatively, the use of an additional crosslinker can be avoided through the incorporation of suitable complementary functionalities within the polymer for latent crosslinking reactions. This approach has been described in U.S. Pat. No. 4,812,541, using N-vinyl lactam and glycidyl monomers. Similar pendant functional group-containing polymers are also described in U.S. Pat. Nos. 4,908,229, 5,122,567, and 5,274,063. These methods typically require the use of solvent for coating. Hot-melt coating offers advantages over these techniques both in terms of economics and environmental impact.
While these patents describe applications in the area of pressure sensitive adhesives, similar strategies have been used in a number of other applications. U.S. Pat. No. 4,678,846 describes a radiation curable release coating, or low adhesion backing, incorporating acrylate functionality on polydimethylsiloxane polymers. U.S. Pat. No. 5,804,301 describes a radiation curable coating suitable as an ink receptor, including a polyfunctional acrylic oligomer of low molecular weight. U.S. Pat. No. 4,798,852 describes a radiation-curable coating for optical glass fiber. In each case, these methods rely on subsequent treatment with radiation in order to effect crosslinking. In general, the effectiveness of the crosslinking is affected by the thickness of the coating. Thermal cures can thus offer advantages over crosslinking from non-thermal energy sources. However, thermal curing requires that the crosslinking be performed subsequent to coating, particularly when hot-melt coating is the method of choice.
Polymers can be applied as coatings using other methods, through appropriate choice of physical properties, such as the glass transition temperature Tg. For example, as described in U.S. Pat. No. 5,948,866, powders of moderate Tg (˜40° C.) can be applied to a surface through electrostatic coating, and subsequently cured to form a uniform layer. This process is similar to hot-melt processing, in that no solvent or plasticizer is required, and is thus advantageous for economic and environmental reasons.
Alternative polymer backbones may also be of interest. In addition to the siloxane-containing materials described previously, fluorocarbon polymers are of interest due to their unique surface energy characteristics and chemical resistance, as well as their oleophobic and hydrophobic character. Polymer fluorocarbons are of great commercial interest in the area of stain and water repellency. Alternatively, polymers with great temperature resistance, such as polyaromatic compounds or cyclic olefin compounds, such as norbornenes, can be useful in specific applications. In general, these materials suffer from poor processing characteristics, such as very high viscosity at high molecular weight or poor solubility in common solvents. There is thus an opportunity for increased use of these materials if these processing limitations can be overcome.
In some cases, enhanced processability can be afforded by branched systems. Branched molecules can act to increase viscosity, through the presence of long-chain branches, or reduce viscosity, through a very high degree of short chain branching. As discussed in U.S. Pat. No. 5,726,249, the presence of branching is also known to enhance wear and chemical resistance in clear coats. Functional branched reactive polymers are thus useful in creating materials with enhanced mechanical and processing properties.
In certain situations, it can also be advantageous to have a mechanism of altering the properties of a coating or article during manufacture. This change in properties can be addressed through the use of a dual-cure system, permitting an initial network to be formed on the basis of one coreactive pair of functionalities. The crosslink density can subsequently be enhanced through reaction of a second pair of functionalities, as described in U.S. Pat. Nos. 5,804,657 and 5,907,024. This control of crosslink density is of much interest in the current art.